Sub-ambient cooling is conventionally accomplished through gas/liquid vapor phase compression based refrigeration cycles using Freon type refrigerants to implement the heat transfers. Such refrigeration systems are used extensively for cooling human residences, foods, and vehicles. Sub-ambient cooling is also often used with major electronic systems such as mainframe server and workstation computers. Though vapor compression cooling can be very efficient, it does require significant moving hardware, including at a minimum, a compressor, a condenser, an evaporator, and related coolant transfer plumbing. As a result of the complexity, associated high cost, and lower reliability, vapor compression cooling has not found material acceptance in small cooling applications, for example personal computers.
The fact that CMOS logic can operate materially faster as the temperature decreases has been well known for at least ten years. For example, if CMOS logic devices are operated at -50.degree. C., the performance is improved by 50 percent over room ambient temperature operation. Liquid nitrogen operating temperatures, in the range of -196.degree. C., have shown 200 percent performance improvements. Similar benefits have shown to accrue for integrated circuit wiring, where metal wiring resistances decrease by a factor of 2 for integrated circuits operated at -50.degree. C. in comparison to room ambient operation. This improvement rivals the recent technological breakthrough of using copper wiring in integrated circuits to reduce interconnect resistance and thereby effectively increase the operating frequencies attainable. Thus, sub-ambient operation of integrated circuit logic devices, such as field effect transistors, as well as the interconnect wiring can materially improve the integrated circuit performance, leaving the question of how to accomplish such cooling in the confines of an ever decreasing size and materially shrinking cost environment.
Thermoelectric cooling is one alternative that has found some usage given the compact size of the prevalently used Peltier devices. A Peltier device is fabricated from semiconductor material such as bismuth telluride or lead telluride. Though new materials are now being evaluated in various universities, they have yet to reach fruition. The commonly used Peltier materials exhibit very high electrical conductivity and relatively low thermal conductivity, in contrast to normal metals which have both high electrical and thermal conductivity. In operation the Peltier devices transport electrons from a cold sink, at temperature T.sub.cold, to a hot sink, at temperature T.sub.hot, in response to an electric field formed across the Peltier device.
FIG. 1 schematically depicts a conventional Peltier type thermoelectric element (TE) 1 with DC power supply 2 creating the electric field across TE 1 while at a load current 3. The desired heat transfer is from cold sink 4, at temperature T.sub.cold, to hot sink 6, at temperature T.sub.hot. As indicated in the equation of FIG. 1, the net heat energy transported is composed of three elements, the first representing the Peltier effect (thermoelectric) contribution, the second defining negative Joule heating effects, and the third defining negative conductivity effects. The thermoelectric component is composed of the Seebeck coefficient, the temperature of operation (T.sub.cold) and the current being applied. The Joule heating component reflects that roughly half the Joule heating goes to the cold sink and remainder to the hot sink. Lastly, the negative component attributable to thermal conduction represents the heat flow through the Peltier device, as defined by the thermal conductivity of the Peltier device, from the hot sink to the cold sink. See equation (1). EQU q=.alpha.T.sub.cold I-1/2I.sup.2 R-K.DELTA.T (1)
FIG. 2 schematically depicts a conventional thermoelectric cooler (TEC) 200 with DC power supply 208 creating an electrical field across thermoelectric element 201, 202 having n-type and p-type semiconductor materials, respectively, to produce a Peltier effect. During operation, thermal sink 203 absorbs thermal energy while thermal sink 204 dissipates thermal energy for providing thermoelectric cooling to an object. Currently, metal tabs 205, 206, 207 are soldered to the thermoelectric elements providing both thermal and electrical conductivity. In this configuration, a problem occurs in that the distance between thermoelectric elements is not minimized thereby leading to inefficient energy transport. Another common problem that occurs is that the metal tabs and solder reduces energy transport between the thermoelectric elements creating inefficient thermoelectric cooling.
Thus, there are a number of very fundamental constraints on current thermoelectric coolers that have created a need for maximizing transport between thermoelectric elements to thereby increase thermoelectric cooling.